For the continuous separation of a filtrant such as sugar-beet pulp into solids and a liquid filtrate a pocket centrifuge is frequently used. Such an arrangement has a rotor that carries a plurality of so-called sieve pockets, each having a sieve surface that faces generally forwardly and inwardly relative to the rotation direction and an axis of the rotor. A chamber formed behind each of these sieve pockets is provided with a drain through which the filtrate can be withdrawn. The sieve surface forms at any point on its surface a respective sieve angle relative to a perpendicular to a plane including the rotation axis.
The solids resting on the sieve surface have a friction angle which is a function of the type of solids, the type of surface they are resting on, and the force pressing the solids against the sieve surface. This friction angle is normally measured to a perpendicular to the force acting on the solids. When the surface supporting the solids is inclined at an angle greater than the friction angle relative to this force the solids will slide along it. When the surface is at a smaller angle the solids will stand without moving on the surface.
In a centrifuge the force is the centrifugal force which presses the solids radially outwardly. This centrifugal force obviously can be resolved into a force vector normal to the surface and one parallel to the surface. The type of surface and the type of solids establish the coefficient of friction, both static and sliding, that together with the magnitude of the force establish the friction angle. As the force increases the friction angle increases, assuming the coefficient of friction remains the same.
Accordingly it is a standard practice to operate the pocket centrifuge at such a rate that the friction angle is slightly less than the sieve angle, so that the solids will move radially outwardly on the sieve pockets. Since the filtrant is normally charged onto the sieve pockets at the radial inner and leading edges thereof, relative to the axis in the rotation direction of the rotor, this means that the solids will slowly slide in a layer radially outwardly along the sieve surface. During such radial outward displacement the liquid in these solids is, of course, driven through the sieve surfaces where it can be recovered separately. At the outer edges of the pocket the solids are typically cast radially off into a solids collection area.
It is known, as for example from German patent document No. 1,119,775 filed Sept. 8, 1959 by H. PINKOW, to mount the various pockets on the rotor so that they can be pivoted on respective pocket axes. Thus it is possible in the system to alter the sieve angle, even during operation of the centrifuge. This publication describes that such adjustment is carried out to optimize the centrifuge operation so as to achieve a steady-state output. Another such centrifuge is described in German patent publication No. 1,183,444 filed Mar. 21, 1961 by J. von Rotel and M. von Rotel.
So-called wobble centrifuges are also known which have a single normally conical perforated drum that is simultaneously rotated about a central axis and caused to wobble about this axis. The rotation separates the filtrate and solids centrifugally, and the wobbling causes the solid to migrate along the drum surface. Rotation speeds, both for simple rotation and wobbling, however, remain constant during the entire operation of the machine. What is more only a relatively small throughput is possible with such a machine compared to the large throughput of a pocket centrifuge. Such wobble centrifuges can be seen in German patent document No. 941,478 of S. Kiesskalt and F. Kleinlein, No. 1,288,990 filed July 27, 1965 by G. Hultsch and No. 1,757,316 filed Apr. 25, 1968 by G. Grimwood.